Aircraft Nuclear Propulsion - Technology

Technology

The principles behind using atomic energy for the propulsion of
aircraft were developed early in the atomic age. As early as 1942
Enrico Fermi and his associates involved with the Manhattan District
Project discussed the use of atomic power to propel aircraft.[1] It was in 1946 that a study by John
Hopkins University's Applied Physics Laboratory delineated the
potentials and problems of using atomic power for aircraft
propulsion. Chief amongst the problems at the time was the lack of
data on the effects of radiation on materials which would be used in a
design.[2] Some of the other basic
problems were the possible release of radioactive fission products or
isotopes during normal operation or due to any accident, shielding the
crew and persons on the ground from radiation, and the selection of
test sites and ranges. There was the potential for the release of
radioactive materials to the atmosphere and the problems of direct
radiation during operational use.[3]
The requirements for an operational nuclear aircraft were that, even
under the most adverse conditions, the aircraft did not add materially
to the general background atmospheric radioactivity and that while in
use the aircraft restricted all harmful radiation to within the craft
or a predesignated exclusion area.[4]

In 1946 the interest in atomic aircraft developed into a long-lived
project know as NEPA, for Nuclear energy for the Propulsion of
Aircraft.[5] The NEPA project, which
started in May, was controlled by the United States Air Force (USAF)
and was therefore oriented towards developing both an atomic- powered
long-range strategic bomber and high-performance aircraft. Nuclear
power showed promise in both fields because of its dual nature of
long-lasting fuel supply and the high temperatures theoretically
possible using a reactor. However, in a paper in 1957 Kelly Johnson
and F. A. Cleveland, both of Lockheed Aircraft Corporation, wrote,
"It appears that the strategic bomber, by requiring both high
speed and great endurance and because of the inherent low-altitude
potential advantages over similar chemical airplanes, will be the
first candidate for a nuclear power plant."[6]

The NEPA contract was with the Fairchild Engine & Airframe Co.,
and the work was conducted at Oak Ridge. By the end of 1948 the USAF
had invested approximately ten million dollars in the program.[7] Extensive studies were conducted under
NEPA from 1946 until 1951, at which time it was replaced by the joint
Atomic Energy Commission (AEC) / USAF ANP program. The ANP program set
forth the ambitious goal of full-scale development of aircraft reactor
and engine systems. One of the factors that led to the creation of the
ANP program was a study done at MIT by a group convened by the AEC in
1948 to look at the potential uses of atomic powered flight. "This
study group, known as the Lexington Project, came to the conclusion
that nuclear aircraft (manned) were likely less difficult than nuclear
ramjets, which, in turn, would be less difficult than nuclear rockets
to develop."[8] Ironically, this turned
out to be the reverse of the proper order of difficulty, as later
research and development would prove. Although nuclear ramjets, under
Project Pluto, and nuclear rockets, under Project Rover, were
successfully tested at the levels needed for operational use, an
operational level atomic aircraft powerplant was never developed. In
1954, Raymond Clare Briant, who was then the director of the ANP
Project stated that "manned nuclear aircraft pose the most difficult
engineering development job yet attempted within this century."[9]

Unfortunately the ANP program wasn't very well organized. Instead
of trying to develop one aspect of the technology to a working stage
the effort was spread out over a number of areas. Part of the problem
was that, under the conventional guidelines, the AEC was responsible
for reactor development while the Air Force was responsible for
development of the remainder of the system. Therefore the project was
divided into two parts which needed to work closely together, but
these two parts were managed by totally separate entities.

Under the ANP program the General Electric Co., at Evendale,
Cincinnati was issued a contract to develop a direct-cycle turbojet,
and Pratt & Whitney Aircraft Division of United Aircraft Corp. was
authorized to study an indirect cycle and work was started at the
Connecticut Aircraft Nuclear Engine Laboratory (CANEL).[10] In the direct air cycle air enters
through the compressor stage of one or more turbojets. From there the
air passes through a plenum an is directed through the reactor
core. The air, acting as the reactor coolant, is rapidly heated as it
travels through the core. After passing through the reactor the air
passes through another plenum and is directed to the turbine section
of the turbojet(s) and from there out through the tailpipe.[11] An indirect system is very similar,
except that the air does not pass through the reactor itself. After
passing through the compressor the air passes through a heat
exchanger. The heat generated by the reactor is carried by a working
fluid to this heat exchanger. The air then passes through the turbine
and out the tailpipe as above. The working fluid in the indirect cycle
is usually a dense fluid, such as a liquid metal, or highly
pressurized water. This allows more heat energy to be transfer,
thereby increasing the efficiency of the system. [12]

In an article in the SAE Journal, L.W. Credit wrote, "Of three
alternatives for achieving flight reliability in nuclear aircraft
through component or system redundancy, the single-reactor,
all-nuclear aircraft seems to be the optimum design." [13] The other two alternatives were a
dual-reactor system and a combination nuclear-chemical (combustion)
system. Originally the ANP program was to develop an indirect cycle,
single reactor propulsion system. However, a petition by General
Electric to the government allowed them to develop the direct cycle
system. GE claimed that the direct cycle was simpler and therefore
would have a shorter development time. For the indirect cycle system,
Pratt & Whitney developed the super-critical water reactor, in
which the working fluid is water heated to 1,500 degrees fahrenheit,
but kept in a liquid state by pressurizing to 5,000psi. This avoided
the problems of using a liquid metal working fluid. The United States
has never favored the operational use of liquid metal reactors. To
date all military reactors in active service, with the exception of
the one liquid sodium reactor on the attack submarine USS Seawolf,
have been of the Pressurized Water Reactor (PWR) type. Even the USS
Seawolf experienced enough problems that the liquid sodium reactor was
replaced with one of a PWR design after a few years in service.

Part of the ANP program was the X-6 program. Beginning in 1952, the
designated goal of the X-6 program was to produce two flying testbeds
powered by atomic energy. The test program started by testing
shielding problems. A B-36 was converted for this purpose. This
aircraft was referred to as the Nuclear Test Aircraft (NTA). The NTA
began its life as a Convair B-36H bomber, but after conversion it was
redesignated as an NB-36H. It was modified to carry a small air cooled
reactor in the aft bomb bay and to provide shielding for the crew. The
NTA incorporated shielding around the reactor itself and a totally new
nose section which housed a twelve ton lead and rubber shielded
compartment for the crew. There were also water jackets in the
fuselage and behind the crew compartment to absorb radiation. The
reactor was made critical in flight on several occasions and the
aircraft was used for many radiation and shielding experiments.

Convair's successful flight program with the B-36
carrying a flight test reactor (July 1955 - March 1957)" showed
that the "aircraft normally would pose no threat, even if flying
low. The principal concerns would be: (a) accidents which cause the
release of fission products from the reactors, and (b) the dosage from
exposure to leakage radioactivity (in the direct cycle concept).[14]

It was decided that the risks caused by radiation were no greater
than the risks that had been incurred during the development of steam
and electric power, the airplane, the automobile, or the rocket.[15]

The B-36 was also to provide the basis for the actual X-6 aircraft.
At the time the B-36 was the only existing, time tested, airframe
large and powerful enough to carry the expected engine and shield
weight. The engine chosen was the J53 turbojet.[16] At the time the J53 was a
conventional turbojet in the planning stage at General Electric. The
J53 was a high- performance design and it was felt that conversion to
nuclear power would present no more difficulty than any other design
then in use. In the early stages of the program, before GE's petition,
it was planned to connect the J53 to a liquid-metal reactor for use on
the X-6. The original propulsion system was to have weighed 165,000
pounds. This was composed of a 10,000 pound reactor, 60,000 pounds of
reactor shielding, 37,000 pounds of crew shielding, and a total engine
weight of 18,000 pounds plus an additional 40,000 pounds for ducts and
accessories.[17] After experiencing
development problems with the J53, GE resorted to the J47 as the
powerplant. J47s converted for nuclear testing were referred to as
X-39s.

It should be noted that the United States was not the only country
working on atomic aircraft in the early years. The Soviet Union had a
few projects of their own. One aircraft, a flying boat, proposed in
1950 would have had a flying weight of 1,000 tons.

It was planned to equip the giant airplane with four
atomic turbo-prop engines. The wing span was more than 130 meters, and
the total power of the engines exceeded one-half million horsepower.
This airplane was supposed to carry 1,000 passengers and 100 tons of
load at a speed of 1,000 kilometers per hour.[18]

It was planned to surround the reactor with five layers of
shielding. The layers were supposed to be as follows: first layer -
beryllium oxide reflector; second layer - liquid sodium for removing
heat from the walls; third layer - cadmium, for absorbing slow
neutrons; forth layer - paraffin wax, for slowing down fast neutrons;
fifth layer - a steel shell, for absorbing slow neutrons and
gamma-rays. Such multilayer 'armor' permits decreasing the weight and
size of the necessary shielding. The coolant was liquid lead.[19]

The Soviets studied many of the same options the United States
considered; both direct and indirect cycles, turbo-props, shadow
shielding, and the special ground handling needed. One fact that is
striking is that in the Soviet design the total weight of the atomic
power plant was to be 80 tons.[20] 80
tons is equal to 160,000 pounds, which compared to the original
figures for the X-6 propulsion system, which was 165,000 pounds, was
practically identical.

The reference to 'shadow shielding' above is to the practice of
dividing the shields between the reactor and the crew, the crew being
in the 'shadow' created by the shields. This is also referred to as
the divided shield concept.

If it were possible to put as much shielding on the
reactor as is done on ground reactors, we could reduce the radiation
therefrom to a negligible amount. But the total weight of shielding
required to do this would be prohibitive; in fact, we are forced to
the so-called 'divided shield' concept in order to reduce total shield
weight to an acceptable amount. Divided shielding is, of course,
simply a division of the shielding between the reactor and the crew
compartment in such a fashion as to result in near- minimum total
shielding weight.[21]

Distributing the shields lessens the total shield weight, but it
also means that the majority of the aircraft would have been exposed
to higher levels of radiation. And once on the ground more radiation
would penetrate the surrounding area. These problems were to be
overcome by newer materials and by designing the aircraft's servicing
equipment with the higher radiation levels in mind. Divided the
shields also had some other benefits;

The directional nature of the radiation leads also to
the fact that aircraft structure and components are useful as
shielding material, and judicious use of such things as the wing box,
landing gear, pay load, and fuel for landing go-arounds can reduce the
thickness of shielding required on the crew compartment rear face.[22]

The problem with shield weight was one of two major problems which
surfaced during the program. The other was increasing reactor
performance. The ANP program focused a great deal of effort on
developing the divided shield concept, decreasing the required shield
size by decreasing reactor size via increasing reactor power density,
increasing the operating temperature of the reactor to boost
efficiency and therefore aircraft performance, and utilizing the
reduced shield mass in aircraft design.[23] Although work on an actual airframe
never got very far, a great deal of work was accomplished on the power
plants.

General Electric ran a series of very successful experiments using
the direct cycle concept. These were referred to as the Heat Transfer
Reactor Experiment (HTRE) series. The series involved three reactors,
HTRE-1 through HTRE-3. HTRE-1 became HTRE-2 at the conclusion of its
test program. HTRE-1 (and therefore HTRE-2) successfully ran one X-39
(modified J-47) solely under nuclear power. HTRE-3 was the closest to
a flight article the program came. It was solid moderated, as opposed
to the earlier reactors which were water moderated, and it powered two
X-39s at higher power levels. HTRE-3 was limited by the two turbojets,
but it could have powered larger jets at even higher power levels.
HTRE-1 was principally a proof of concept reactor. "HTRE-1
achieved a number of full-power runs that demonstrated conclusively
the feasibility of operating a jet engine on nuclear power."[24] HTRE-2 was simply HTRE-1 modified
to test advanced reactor sections in a central hexagonal chamber. In
this way new reactor designs could be tested without the need to build
a totally new reactor from scratch. The experience gained from HTRE-1
and HTRE-2 was used in the construction of HTRE-3. HTRE-3 was the
final test item designed to prove the feasibility of producing an
actual aircraft powerplant. "The design and testing of HTRE-3
has advanced the direct-cycle program beyond the question of
feasibility to the problems of engineering optimization."[25]

All three of the HTRE reactors were of the standard direct cycle
configuration, with the addition of a chemical combustor just upstream
from the turbines. This combustor allowed the jets to be started on
chemical power and then be switched over to atomic heat as the reactor
was brought up to operating temperatures. The operational system may
have also utilized a chemical combustor for use during takeoff and
landing, and possibly target penetration, when the reactors relatively
slow response time could be a disadvantage.[26]

The HTRE either met or exceeded their goals, but although all had
reactor cores of roughly the size needed to fit into an aircraft, none
of the HTREs were designed to be a prototype of a flight system[27]; the series showed that it then
appeared "possible and practical with the technology in hand to
build a flyable reactor of the same materials as HTRE-3 and similar in
physical size."[28] Despite the
fact that HTRE-3 didn't produce the power that would have been needed
for flight, that was mainly because it was not an
optimized design; it was designed simply as a research reactor, to
prove the concepts needed for a flight article.

At the end of the HTRE run the probability of flying a reactor
seemed high. The test runs showed that a reactor using the same
materials as HTRE-3, and which could power a gas-turbine powerplant,
could have been built at that time. Such a reactor would meet all of
the requirements needed for a flight ready unit.[29] In their paper Kelly Johnson and
F. A. Cleveland also stated that "when improved materials are
available, we would expect the nuclear power plant to advance rapidly
in its overall efficiency, with a consequent improvement in ability to
install such power plants in airplanes of smaller size than those
currently contemplated."[30]

While GE was working on the direct cycle, Pratt & Whitney
(P&W) was working on the indirect cycle. However, progress went
much slower that it did with the HTREs. P&W never ran a practical
test system. In fact their work was limited to component testing. In
addition to work on the super-critical water reactor P&W worked
with liquid metal coolant designs. It was the latter that received the
most attention. The two major designs were a solid core reactor, in
which the liquid metal circulated through a solid reactor core, and a
circulating-fuel design, in which fuel was mixed with the coolant and
critical mass was achieved as the coolant circulated through a central
core. After the circulating-fuel design showed promise, work on the
super-critical reactor was halted. P&W did accomplish a great deal
on the design of liquid metal cooling loops, corrosion prevention, and
heat exchanger design. However, P&W work at CANEL never led to a
test reactor, much less one which was flight ready. In the long run
the indirect cycle showed more promise, but it also required a great
deal more developmental work.

While these test programs were successful, there were other
programs which weren't. A number of programs were begun at a great
cost of time and money, only to be dropped when the program went
through one of its many reorientations. The official U.S. government
report on the ANP project lists such programs. A Flight Engine Test
facility was built in Idaho for use to test the flight engine both on
the ground and in the test aircraft. This facility cost over eight
million dollars, yet it was never used during the ANP program, other
than as a storage building, because the flight program was
cancelled. A radiator laboratory was constructed at CANEL for use in
studying liquid metal to air heat transfer. After spending over six
million dollars the construction was halted with only a shell
completed because the Air Force changed its mind. Another laboratory
was built at CANEL to study vacuum conditions. This laboratory cost
over a million dollars, and it entered use in March 1961, the same
month that the ANP program was cancelled. These were only the largest
of the wastes. There were numerous instances of wasted time and money,
none of which can really be blamed on the technicians, since the
leaders changed their minds and the equipment went unused.

Overall the technology seems to have been there, yet the ANP
program died. GE's HTRE series proved that the direct cycle concept
would work. P&W was making progress, slowly but surely, on the
indirect cycle. The NTA reactor tests demonstrated that aircraft
shielding could be done effectively. A myriad of smaller developments,
new metals, synthetic lubricants, all worked out and were available to
produce an aircraft. In 1960 Kenneth Gantz wrote, "The taming of
the atom, coupled with the technological advances in aerodynamic and
structural efficiencies achieved over the past several decades, now
brings atomic-powered aircraft and missiles within our grasp."[31] But if it wasn't killed by
technology, what were the reasons for the program's demise? The answer
to that question follows in the next section.